Ultrasonic flow measuring devices are applied often in process and automation technology. They enable contactless determination of the volume and/or mass flow rate of a medium in a pipe.
The known ultrasonic measuring devices work either by the Doppler principle or the travel-time-difference principle. In the case of the travel-time-difference principle, the different travel time of the ultrasonic measuring signals in the direction of flow, and counter to the direction of flow, of the medium is exploited. To this end, the ultrasonic measuring signals are alternatingly issued, respectively received, in the direction of flow, and counter to the direction of flow, of the medium. On the basis of the travel-time-difference of the ultrasonic measuring signals, the flow velocity can be determined, and, with that and known diameter of the pipe, the volume flow rate of the medium, or, with known density, the mass flow rate of the medium.
In the case of the Doppler principle, ultrasonic measuring signals of known frequency are coupled into the flowing medium. The ultrasonic measuring signals reflected in the medium are evaluated. On the basis of a frequency shift occurring between the ultrasonic measuring signal which was coupled into the medium and the reflected ultrasonic measuring signal, likewise the flow velocity of the medium, or the volume and/or mass flow rate, can be determined. The use of flow measuring devices working according to the doppler principle are only possible, when are air bubbles or impurities are present in the medium, on which the ultrasonic measuring signals are reflected. Thus, the use of ultrasonic flow measuring devices using the Doppler principle is rather limited, compared to ultrasonic flow measuring devices using the travel-time-difference principle.
With respect to types of measuring devices, a distinction is drawn between ultrasonic flow measuring pickups, which are inserted into the pipeline, and clamp-on flow measuring devices, where the ultrasonic transducers are pressed onto the pipeline externally by means of a clamp. Clamp-on flow measuring devices are described, for example, in EP 0 686 255 B1, U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593.
In the case of the two types of ultrasonic flow measuring devices, the ultrasonic measuring signals are radiated at a predetermined angle into, and/or received from, the pipe containing the flowing medium. In order to be able to radiate the ultrasonic measuring signals at determined angles into and out of the pipe, and into and out of the medium, as the case may be, the in- and out-coupling of the ultrasonic measuring signals into and out of the pipe occurs in the case of clamp-on flow measuring devices via interface pieces, or coupling wedges. In order to achieve an optimum impedance matching, it is, moreover, known to make the coupling wedges of a suitably refracting material, e.g. a synthetic material, or plastic. The principal component of an ultrasonic transducer is usually at least one piezoelectric element, which produces the ultrasonic measuring signals and/or receives them.
Now, there are different reasons why the measuring of volume, or mass, flow rate by means of ultrasonic measuring signals may fail. Generally, a bad measurement occurs, however, at least always when the sound path, on which the ultrasonic measuring signals propagate from the sending ultrasonic transducer, to the receiving ultrasonic transducer, gets interrupted at some location. Such an interruption can occur at various locations in the sound path. As examples, the following system and process defects can be mentioned:
The damping of the medium is too great;
the coupling of at least one ultrasonic transducer onto the containment is insufficient;
there is an air gap between the inner wall of the containment and a liner applied to the inner wall of the containment;
the damping in the material of the containment is too great; this can be the case e.g. when the ultrasonic measurement occurs in GFK (glass fiber reinforced plastic) pipes;
the containment, e.g. pipe, is empty—for some reason, there is no medium in the pipe.
It is desirable in this connection that the operating personnel not only be shown that a malfunction has occurred, but, also, where the real cause of the malfunction is to be sought. In the known flow measuring devices, the cause of an occurring malfunction, or the interruption of the sound path, must be determined in a more or less complicated, trial-and-error method. This is, naturally, involved and cost-intensive. Thus, it can happen, that the measuring device is subjected to an extensive examination, when, in fact, the bad measurement is being caused by an absence of medium flowing in the pipe.